feat(frontend): replace emoji with inline Material Symbols icon system

Add a self-contained icon system: vendored Material Symbols registry, an
<Icon> component that references a single <symbol> sprite via <use>, plus
IconButton/IconSprite. Replace every emoji/Unicode glyph (logo, theme
switcher, cover placeholder, like/comment counts, lightbox controls, back
links) and add leading icons to key controls. Decorative icons are
aria-hidden; icon-only buttons get labels; theme toggles expose
aria-pressed; counters get accessible names.

Recorded in openspec change material-icons-ui (all tasks complete).

frontend/src/lib/components/Uploader.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { auth } from '$lib/stores/auth.svelte';
+	import Icon from './Icon.svelte';
 	import type { Photo } from '$lib/types';
 
 	interface Props {
@@ -96,13 +97,15 @@
 		class="hidden"
 		onchange={onChange}
 	/>
+	<Icon name="cloud_upload" class="text-text-subtle mx-auto mb-3 size-10" />
 	<p class="text-text mb-3 text-sm">Перетащите фото сюда или</p>
 	<button
 		type="button"
 		onclick={pick}
 		disabled={busy}
-		class="bg-primary text-primary-fg rounded-lg px-4 py-2 text-sm font-medium disabled:opacity-50"
+		class="bg-primary text-primary-fg inline-flex items-center gap-1.5 rounded-lg px-4 py-2 text-sm font-medium disabled:opacity-50"
 	>
+		<Icon name="add_photo_alternate" class="size-4" />
 		{busy ? 'Загрузка…' : 'Выбрать файлы'}
 	</button>